Controllers, such as programmable logic controllers (PLCs), are used in a wide variety of environments to perform various control functions, such as sensing parameters and providing output signals to various control devices, often based on the sensed parameters. One class of programmable controller is the micro-controller which includes inputs, outputs and a power supply in a single integrated package. A typical micro-controller has relay outputs that utilize relay coils to actuate the relay outputs.
Micro-controllers often are used in environments where it is beneficial to reduce the size of the packaging to the extent possible. It also is advantageous to maximize any reduction in heat generated by the micro-controller, particularly at the relay coils. Generally, the relay coils utilize a relatively large amount of energy, sometimes referred to as power dissipation, that creates undesirable heat. Furthermore, the voltage required to energize a given relay coil is higher than the voltage necessary to hold or maintain the relay coil in an energized state. Thus, unless the voltage at a given relay coil can be reduced, after initial energization, to a level just sufficient to maintain the coil in its energized or actuated state, excess power is generated and lost by the energized coil, largely in the form of unwanted heat.
Attempts have been made to reduce the voltage at a given relay coil after energization. For example, in some relay systems a resistor and capacitor are connected in parallel between the power supply and each relay coil. This arrangement provides an initial higher energization voltage with a lower voltage thereafter. However, in a micro-controller with numerous relay outputs, the multiple resistors and capacitors present size constraints. Additionally, the heat generated by the many resistors during both energization and the hold period contributes to the unwanted heat problem.
It would be advantageous to have a programmable controller able to utilize two different voltage levels in energizing and holding selected relay coils without the use of multiple resistors and capacitors.